Brief Description of Prior Art
Many technically informed personnel today regard leaks from underground chemical storage tanks as the most prevalent source of drinking water contamination. In a recent study by the Environmental Protection Agency of the United States, it was estimated that there are approximately 800,000 individual motor-fuel storage tanks, including tanks for storage of gasoline and diesel oil, located in about 325,000 establishment locations in the United States. The study moreover indicated that at least twenty five percent of these tank systems had significant leakage when the pressure in them was slightly elevated. The average rate of leaking from these tanks was about 0.31 gallons per hour, with half of the leaks being about 0.25 gallons per hour or less. Fiberglass tanks appeared to have failure rates approximately as high as the failure rates for steel tanks. About twenty one percent of the motor-fuel storage tanks are installed, partly or completely below the water table in the ground.
In the case of steel tanks, it is a widespread practice to protect such tanks against corrosion by electrically isolating connecting piping of dissimilar metals to prevent the development of corrosive currents, by protectively coating the inside and outside of the steel tank with a dielectric coating material, by means of cathodic protection which utilizes a sacrificial anode in order to reverse corrosive underground current flow and by placing an impressed current on the tank. Such corrosion protection systems do limit, but do not entirely eliminate the deleterious effects of corrosion.
The Steel Tank Institute in the United States has developed certain standards for double containment, atmospheric-type rigid steel vessels for containing petroleum-derived products, such as gasoline and diesel oil, or for containing hazardous chemicals. The dual or double-walled tanks generally include an outer steel wrap which extends over the predominant portion of the lower circumference of the internal tank. The outer wrap or tank is welded to the inner tank only at the upper perimeter of the outer wrap or tank where it is joined to the inner tank to make the secondary containment which it affords liquid tight. A system for monitoring the interstitial space between the outer wall and the inner wall has been proposed which includes use of vacuum developed in this interstitial space. Before installation, the inner tank is tested at a positive pressure and excessive atmospheric pressure to determine the presence of leaks. At the same time that the positive pressure is applied internally in the inner tank, equivalent positive pressure is developed in the interstitial space between the outer wrap and the inner tank. All visible seams and welds are then covered with a soap solution for the purpose of detecting leaks.
Fiberglass tanks do not suffer from the effects of galvanic corrosion, and are inert to some corrosive stored liquids. A problem which has been encountered with fiberglass underground storage tanks, however, is that unless they are loaded perfectly for transport, the vibration and jolting encountered can cause these tanks to develop cracks before they even arrive at the installation site. Such tanks have been known to shatter when lifted improperly, or when dropped during installation. The site itself must be prepared to exacting specifications to prevent uneven stresses from developing which will later cause a fracture in the tank following installation.
It is known to place flexible or semi-rigid liners within metallic liquid storage tanks in order to protect the tank from corrosive fluids to be stored therein. Thus, Beuglet, U.S. Pat. No. 2,762,736 proposes to line a metallic tank with a plastic liner, adhering the liner to the tank's internal wall by means of evacuation through a fitting providing communication with the interior of the tank.
The liner of Beuglet protects the tank from corrosion. Moreover, fluids can be contained in metal tanks without undergoing metal contamination. A wire netting or grid can be interposed between flat plastic plates formed on one side of the liner and the tank wall so as to permit the vacuum to act over the entire exposed surface of the liner and draw it uniformally into contact with the wire netting when the air is evacuated from the intervening space. The tank is provided with a manhole cover for covering the main opening into the tank, and the means used to secure the manhole cover in position is also utilized, in part, to secure the internal liner within the tank.
The system depicted and described in the Bueglet patent makes no provision for continuously monitoring the integrity of the system to determine whether the liner has developed a leak, or remains satisfactorily in tact, and for intermittently evacuating the space between the liner and tank. Such a monitoring system is provided in the present invention, and the system by which the space between the liner and the tank is evacuated has also been improved relative to that depicted in the Beuglet patent.
Switzer, U.S. Pat. No. 2,847,959 is also concerned with the placement of a flexible liner within a metallic container. The space between the liner and the container is evacuated so that the liner is expanded and held in intimate contact with, the wall of the container. In the Switzer patent, the hole or space through which the air is drawn out and the space evacuated is subsequently sealed after the liner has been drawn into contact with the internal wall of the container. Once this opening has been sealed, there is no further opportunity to restore the vacuum between the liner and internal wall of the container, or to monitor the integrity of the liner, except by visual inspection of the interior of the container at periodic intervals.
Gray, U.S. Pat. No. 2,346,423 is concerned with lining a steel tank with a malleable metal, such as copper. In order to force the copper liner into intimate contact with the entire internal surface of a surrounding steel tank, the liner is filled with water to a test pressure of about 300 p.s.i., and the air in the space between the vessel or jacket and the liner is concurrently evacuated. Although there is provision for measuring the extent to which the space between the copper liner and the steel tank has been evacuated, once the evacuation operation is completed, the fitting to which this gauge is connected, and in fact, to which the vacuum pump has been connected, is sealed off permanently, and there then follows no further vacuum testing by means of a gauge connected at this point.
Arne, U.S. Pat. No. 3,064,344 proposes to place a flexible metallic liner inside a steel vessel. A vacuum is drawn between the liner and the tank shell to pull the liner out against the shell and keep the liner from buckling. The metallic liner will present the disadvantage of being reactive with some types of chemicals stored in the tank.
U.S. Pat. No. 3,942,331 to Mewman, Jr. et al. discloses a cryogenic tank for holding cryogenic liquids. A relatively thin sealing membrane is provided inside a housing or other rigid vessel, and the space between the sealing membrane and the rigid outer wall of the vessel is filled with an insulating material. The insulating material is said to define a plurality of ducts or conduits between the insulating material and the outer wall of the vessel. These ducts or conduits can receive gases indicative of leakage from the tank and transmit these gases through an external conduit system to a gas detector device.
The Mewman system does not appear to propose or offer any sort of arrangement for continuously monitoring the vacuum which exists between the relatively thin sealing membrane and the housing or outer vessel.